Solder deposition system

ABSTRACT

A tool employing a solder foot to deposit solder on a series of conductive surfaces as the tool moves. In one embodiment, non-wettable blade attached to the tool breaks the film. A pair of sensors coupled to a control circuit monitor the position of the solder foot and the position may be changed as a function of the operation to be performed, i.e. deposit, reflow, standby while tool is moved. In a second embodiment a discrete solder mass is extruded and deposited on a preheated pad. In a third embodiment, solder wire is delivered to an ominidirectional tool for deposition.

This is a division of application Ser. No. 181,775 filed Apr. 15, 1988now U.S. Pat. No. 4,898,117.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved system of soldering and inparticular to an improved technique of depositing solder onto solderwettable contact pads with a substantially uniform amount on each pad.

II. Description of the Prior Art

Solder distribution onto mounting pads for surface mount boards has inthe semiconductor industry generally been accomplished utilizing ascreening process. In this technique, art work and screens must befabricated having the solder deposition pattern. Then, a precisionalignment process is carried out wherein the solder is screened onto thesurface mount pads. The solder paste used for the process requiressubstantially long cure and bake times. Thus, in addition to thecomplexity of the alignment process, this prior art technique isrelatively time consuming.

The prior art technique utilizing screening is further complicated by arequirement that the pattern mixes very fine lead pitch and widthsurface mount pads along with standard surface mount parts. For example,in the case of tape automated bonding the pitches vary from 4 to 20 milswhile, the standard surface mount parts have pitches in the range of 20to 50 mils. Thus, the process requires that different amounts of solderbe distributed on various parts of the board. Given the precisionrequired, it is common to utilize separate screening steps, one for veryfine lead pitch and width surface mount parts and second for thestandard surface mount parts. There is the possibility of damaging thesolder deposited in a previous step when multiple screening operationsare carried out. Additionally, screening fine line solder presents aproblem because the solder paste tends to stick in the openings of thescreen as the openings get progressively narrower.

Another problem inherent in prior art screening systems is thedifficulty of performing rework. Once a defective part has been removedthere is no currently available technique to replace the solder on theboard site. The solder which remains on the pads would be variable inthickness since there is no way to control the amount which remains witheach lead of the part as it is being removed. While techniques exist tototally remove the solder after the part has been removed, replenishingby screening an isolated site is not yet feasible.

The prior art is also replete with a variety of techniques to depositsolder across the surface of a printed circuit. Typical techniques areso-called dip-soldering and wave-soldering. Wave-soldering involvespumping a molten solder through a nozzle to form a standing wave. Inthis process the entire side of an assembly containing printedconductors with the leads from the circuit components projecting throughvarious points generally travels at a predetermined rate of speed overthe standing surface of the wave of molten solder. The lower surface ofthe assembly is placed into contact with the upper fluid surface of thewave. By this technique, the solder wave in the first instance wets thejoining surfaces and promotes through-hole penetration. This in turnhelps to assure the formation of reliable solder joints and fillets.Wave soldering is illustrated in U.S. Pat. Nos. 3,705,457 and 4,360,144.An example of an immersion technique is illustrated in U.S. Pat. No.4,608,941 wherein panels are immersed in a liquid solder bath and thenconveyed to an air knife which levels the molten solder on the panels.The air knife is therefore used to effectively clear the panels ofexcess solder and only the printed patterns retain the solder.

Another example of a solder leveler is illustrated in U.S. Pat. No.4.619.841. The technique is used in conjunction with dip-solderingtechniques. Other techniques of selective deposition of solder ontoprinted circuit patterns are described in U.S. Pat. Nos. 4,206,254;4,389,771; and 4,493,856.

U.S. Pat. No. 3,661,638 is also directed to a system for leveling andcontrolling the thickness of a conductive material on the walls ofthrough-holes of a printed circuit board. The technique, for removingthe excess amount of conductive material employs heating to melt aconductive material after it has been deposited and then, while theconductive material is in the plastic state, gyrating the board to causethe plastic material to move circumferentially about the through-holeand flow axially through the through-hole.

Summary of the Invention

Given the deficiencies of the prior art it is an object of thisinvention to define a system employing a tool which is usable either byhand or under robotic control that permits a redistribution of reflowedsolder to pads on a surface mount board.

It is a further object of this invention to define a tool which allowsthe deposition and reflow of solder on pads of surface mount boardswithout the requirement for artwork, solder screens, or solder paste.

Yet another object of this invention is to define a system employing atool which will apply solder in a fine line pattern to circuit cardswhile at the same time deposit solder in varying line widths.

A still further object of this invention is to define a tool thatautomatically regulates the amount of solder applied to a circuit cardregardless of the condition of the amount of solder left on the cardafter part removal.

These and other objects of this invention are accomplished by means of atool which deposits onto solder wettable contact pads solder insubstantially uniform amounts on each pad. The tool, one embodimentcomprises a solder reservoir or plenum, a heating element to melt thesolder, and at the bottom of the reservoir, a foot which passes over thecontact pads to be wetted with solder.

The foot employs first and second sensors to control the flow of solderfrom the foot. The foot itself has a forward end for the release ofliquid solder and a back end for holding a shaping element. The footthus has a cavity therein which extends to the bottom and to the forwardend. The cavity is connected to the solder reservoir. At the back of thefoot, a non-solder wettable blade is placed for removing excess materialand providing shape control. The amount of solder flowing out of thefoot is controlled as a function of a vacuum system which works incooperation with the first and second sensors on the solder reservoir.The first sensor is located in the interior of the cavity and is used tomaintain an appropriate level of solder in the cavity in a tool stand-bymode.

The second sensor is disposed at the forward edge of the cavity at thefront portion of the foot. This sensor is employed to control thequantity of solder that resides in the foot during the deposition mode.

In accordance with this invention, solder in this embodiment isdeposited in the following manner. The non-solder wettable substrategenerally has an array of solder wettable contact pads. The foot isbrought onto the substrate and maintained at a sufficient,pre-determined distance above the substrate. The distance is maintainedso that contact to the substrate or the pad does not occur. Solder ispermitted to flow out of the foot by switching to the second sensorwhich monitors the shape of the solder mass emerging from the front ofthe foot. The solder contacts the substrate or pad at the bottom of thefoot. Solder tends to bulge out of the front cavity of the foot as afunction of flow rate. As the foot is moved over the contact pads in thedirection of the front of the foot, the pad is covered with solder. Anon-wettable blade at the back end of the foot breaks the surfacetension of the solder, thus removing the bridge between adjacent pads asthey emerge from the rear end of the tool. The foot can be passed overan array of pads in one direction and then its direction can be changed.The foot will deposit a substantially uniform amount of solder onto eachcontact pad.

In a second embodiment of this invention the tool has a pressuredreservoir and a series of openings on the bottom to produce a discretemass of solder protruding therefrom. A heater is positioned to move withthe tool and is located upstream to preheat the pads. As the tool passesover a pad a discrete drop of solder contacts the preheated pad. Bycontrolling the rate of movement and reservoir pressure, the drop sizeand deposition rate can be determined.

In a third embodiment, the tool is omnidirectional and solder wire isdelivered in metered amounts. A heater in the tool melts the solder toform a liquid mass for deposition. The tool has an omnidirectionalnon-wettable surface to break the surface tension. The solder may beremoved using a vacuum coupled removal pipe when the operation iscompleted.

In all embodiments the tool is computer controlled so that movement overan array of pads can be accomplished. Depending on the technique used toform the solder for deposition, the tool may be unidirectional androtational or omnidirectional.

This invention will be described in greater detail by referring to thedrawings and a description of the preferred embodiments which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of the tool in accordance with a firstembodiment of this invention in the stand-by or stop reflow mode:

FIG. 2 is a side elevation view illustrating the tool in accordance withthe first embodiment in the reflow mode with the foot extended beyondthe leading edge of the head;

FIG. 3 is a schematic perspective view illustrating forward movement ofthe tool;

FIG. 4 is a schematic drawing of a complete instrument used inaccordance with the first embodiment this invention; and

FIG. 5 is a circuit drawing of the control circuit for computer andsensor wires that permits operation of the reflow in accordance with thefirst embodiment of this invention;

FIGS. 6A and 6B, respectively, are side elevation view and bottom viewof a second embodiment of this invention.

FIGS. 7A-7E illustrate configurations of various orifice patterns; and

FIG. 8 is a side elevation view of a third embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1-3, the description of the tool forming a partof this invention will be described. FIG. 1 illustrates the tool in thestand-by or stop reflow mode. A central rod 10 formed of a conductivematerial such as copper is employed as a heat source, An annular outerwall 12 is used as a retaining wall for a solder column 13. Thus, asillustrated in FIG. 1, the liquid solder column 13 is heated by theconductor 10 and maintained by the outer wall 12. This solder column ismaintained in a tool head 14. As illustrated in FIG. 1, the soldercolumn 13 distends about a bottom portion of the conductor 10 to have alower molten solder tip 17. Such is illustrated below the cutaway linein FIG. 1.

A pair of sensors 15 and 16 are used to maintain the shape of thissolder tip. Sensor 15 is an external tungsten wire sensor for the solderfoot in the reflow mode. Sensor 16 is a solder wire utilized in thestand-by mode and as illustrated in FIG. 1, sensor 16 is in contact withthe tip 17 of the solder foot. Sensors 15 and 16 are coupled to thesensor circuit of FIG. 5 to be described herein.

FIG. 1 illustrates the vertically adjustable non-wettable blade 18 whichis coupled to the tool head 14 by a convenient attachment technique suchas a tapped screw or the like illustrated in FIG. 1. Surface 19represents the workpiece.

FIG. 2 illustrates the tool in the reflow mode with the foot 17 nowextended beyond the leading edge of the head to form a protruding zone20. The position of this leading edge 20 is monitored by the sensor 15.This configuration of the solder is different from that illustrated inFIG. 1 where the solder foot is withdrawn having a leading edge 17monitored by the sensor wire 16.

In operation, as illustrated in FIG. 2, the sensor wire 15 monitors thesize of the foot by determining the position of the leading edge 20.FIG. 2 illustrates only a small number of the typical pads 21, 22 and 23on the surface of the workpiece 19. As illustrated, solder 25 whichexists on the pads 21 ahead of the tool has an irregular pattern becausea part may have been removed leaving an irregular amount of solder. FIG.2 illustrates pads 22 having uniform solder deposition 27 after the toolhas passed. As illustrated in FIG. 2, in Zone 23, the solder film isbroken by means of a height adjustable non-wettable blade 18.

FIG. 3 illustrates in schematic view the forward movement of the tool intypical use. As illustrated in FIG. 3, the tool head 14 comprises anopen inverted U-shaped channel 24 defined by the housing. The soldercolumn comprising the conductor 10 and the sheath wall 12 is positionedon the top of the head 14. The foot 20 therefore may extend beyond theopening 24 to wet the mounting pads 21, 21', 21" in succession as thetool moves in the direction illustrated in FIG. 3. Thus, the pads areencountered by the solder foot and are bridged together while under thetool head 14. As the tool head 14 passes over each of the mounting pads,the non-solder wetting blade 18 breaks the solder film so that a uniformcoating exists on each pad.

FIG. 2 illustrates the action of the blade 18. Once the solder film hasbeen broken, the effect of surface tension causes the solder to pullback into the foot 20 and simultaneously, the deposited portion 27 tospread onto the pad 21 in a uniform manner. The blade is verticallyadjustable by manual means (set screw 26) or automatically.Additionally, the blade may be excited by a source of high frequency toprovide vibratory motion. Sonic, piezoelectric or mechanically inducedexcitation may be employed. When the tool reaches the end of a row ofpads. 21, etc., with the solder thus deposited or reflowed, it may beswitched to a stand-by mode by switching from the sensor 15 to thesensor 16. This has the effect of withdrawing the solder foot from theconfiguration illustrated in FIG. 2 to that illustrated in FIG. 1. Thetool may then be turned to address a row of pads at right angles tothose just reflowed or alternatively, to lift the tool in preparationfor moving to another site on the circuit board.

Given the mode of operation, this invention does not require criticalalignment over the pads. The path of travel is typically defined by asolder mask. So long as the tool is roughly positioned over the pads,deposition will occur. That is there is no need to align on the centerline of the row of pads. Also, while not illustrated, a dam may beemployed to confine the flux. The invention thus permits a one passtechnique of solder deposition. While the drawings illustrate atool-down mode of use, the tool may also be employed in a tool-upconfiguration.

Referring now to FIG. 4 the system of this invention is illustrated. InFIG. 4 those elements of FIGS. 1 through 3 which are the same componentsprovided with like numerals. The heat source 10 is coupled to a massheat source 31. As illustrated in FIG. 3 the source 31 is grounded viaground 30. This allows for simple operation of the sensor wires 15 and16 as well as circuit safety of a populated circuit board. Asillustrated in FIG. 4. the solder reservoir container 32 is disposedconcentrically outside the source 31. The container 32 contains a solderreservoir 35 which supplies solder to form the column 13. The solderreservoir 35 is disposed around the heat source 31. The reservoir tapersinto the retaining wall 12.

In order to maintain a sufficient solder foot, pressure on the reservoiris maintained by a gas column 36. The pressure provided by this gascolumn above the reservoir maintains the reservoir position and is thetechnique of applying alternative positive and negative pressure on thesolder to create a solder foot and maintain its position as a functionof that determined by sensor wires 15 and 16. As illustrated in FIG. 4,gas under pressure is admitted from a source 42 through a regulator 38to define a positive gas flow through conduit 37 into the reservoir at aportion 35 above the solder reservoir 35. Similarly, reservoir 39 isused to vent gas and thus define a negative pressure differentialthrough the system.

A solenoid valve 41 is used either to supply positive pressure gasthrough conduit 37 or to define the negative pressure flow for purposesof the venting via regulator 39.

Thus, as illustrated in FIG. 4, depending on the pressure applied inzone 36, the foot of the solder column may be either extended (by theapplication of positive pressure in zone 36) or retracted (by theapplication of negative pressure using regulator 39). Those twoconditions are monitored by means of sensors 15 and 16 which provideinput into control circuit 40 which is used to control the position ofthe solenoid valve 41.

Referring now to FIG. 5, a simple control circuit for computer andsensor wire control of the relay/solenoid system of FIG. 4 is depicted.This circuit is used to control the operation of the reflow tool.

Op-amp A1 is a comparator providing a TTL signal of either 0 or 5 voltsto be used as a computer input to direct the tool to be either in areflow or standby mode. The output also provides activation for sensors15 or 16. This is accomplished by having the transistors TR1 and TR2reverse bias so that one is conducting while the other is not. Such isaccomplished by means of a phase inverter A2 which inverts the output ofOp-amp A1 and directs it to the gate of transistor TR2. One or the otherof sensor wires 15 or 16 is then connected by a low impedance paththrough transistor TR1 or TR2 to the input of comparator A3. The onemegaohm resistor R1 pulls the sensor wire 15 or 16 and inverting inputhigh. The state then reverses when solder touches the wire shorting thehigh voltage low current potential present at the sensor wire andinverting the input terminal to ground. This is reflected by a change instate of the output of comparator A3 resulting in the relay K1 being putin an alternative condition. Relay K1 is then used to drive a highcurrent relay to toggle the solenoid valve 41 of FIG. 4. A pair ofLED's, LED-1 and LED-2 are used to indicate the state of the circuit atany given time.

The values of the various resistor and Zener diodes used in the circuitof FIG. 5 are illustrated. The transistors may be a pair of 2N5465transistors.

In order to confirm that this invention provides results which are atleast comparable with those obtained utilizing conventional solder pastescreening, the following experiment was carried out. 40 samplesconsisted of 6.5×3.5 inch type cards having 10 mil pads on 20 milcenters and 4 mil pads on 8 mil centers for component attachment. Solderapplication was carried out utilizing two methods. The first methodconsisted of the application of solder paste utilizing a thick filmscreen printer. The paste was a Cermalloy 3801 paste and the printer aPresco thick film screen printer Model 8115. This was used to the 10 milwide footprints on 20 mil centers utilizing a 2 mil thick brass stencil.Next, the paste was dried at 120° C. for 10-15 minutes followed by vaporphase reflow of the dry paste at 215° C. for 38 seconds and HTC vaporphase reflow system Model 912W/elev. utilizing fluorinert 5311 fluid.The card was subsequently cleaned in Freon TMS. This process representsa conventional screen solder technique.

The second method of solder application was utilizing the solder foottechnique of this invention. For purposes of this experiment, sensors 15and 16 were omitted. Solder is deposited on the pads utilizing a solderhead of the type illustrated in FIGS. 1-3 containing molten solder. Asthe tool passes over the pads it bridges the pads together. At thetrailing edge of the solder head, a non-wettable blade allows the soldersurface 10 to be broken leaving a well defined amount of solder on eachpad. The amount of solder remaining on the pad is defined by a functionof both height of the non-wettable blade as well as the geometry of thewettable area of the pad. The dimensions of the tool head may be madevery small so as to accommodate narrow sections of populated cards. Theprocedure consists of fluxing the board and then traversing the solderhead across the row of pads manually after adjustments of the head andthe blade.

The speed of travel of the solder foot in these manually prepared boardswas approximately 0.5 inches/second with the solder temperature set tobe 300° C. utilizing an Alfa 102 (unactive) flux. That technique wasdesigned to deposit 2-3 mil solder on 10 mil wide pads and approximately0.5 mil solder on 4 mil wide pads. The solder pads were examined byoptical microscopy. Samples were cross-sectioned and the solderthickness was measured by metallographic technique utilizing an LEC 0300Metallograph. Thickness measurements were made on two sets of parallelpads on each of the cards. High magnification (400× and 1000×) pictureswere taken to show the microstructure, solder profile and intermetallicregions. Measurements of individual solder thickness on two series of 10mil wide pads, mainly screened and reflowed and pads which were solderedutilizing the solder flow technique were tabulated. The table belowprovides averages, standard deviations and maximum/minimum values.

                  TABLE                                                           ______________________________________                                        Parameter of Solder Thickness Measurements (mils)                                       Screen, Past, Reflow                                                                       Solder Foot                                            Parameters  Side A  Side B     Side A                                                                              Side B                                   ______________________________________                                        Average     3.44    3.46       3.84  3.84                                     Std. Dev.   0.16    0.21       0.18  0.08                                     Maximum     3.77    3.89       4.11  4.01                                     Minimum     3.11    2.72       3.33  3.62                                     ______________________________________                                    

As indicated, the two techniques are comparable in terms of averagethicknesses and reproducibility. Both techniques produced a thin layerof intermetallics as expected. The extent of the intermetallics is afunction of the time-temperature profile the pads experience by the twotechniques. In either of the cases, the extent of the intermetallics isnot considered to impact either the solderability or reliability on the10 mil wide pad.

In either of the two pad sizes used, no bridging was observed using thesolder foot technique. Stencil material selection and fabrication,solder bridging and solder particle size are limitations of pastescreening and reflow techniques as footprints become smaller andsmaller.

It can therefore be seen that in accordance with this embodiment of theinvention, a unique tool permits the distribution of reflowed solder topads on a surface mount board. This is accomplished by employing asolder-carrying head with a non-wettable blade whose solder volume ismonitored through a sensor feedback system. The technique may be usedeither by hand or under robotic control. As the head of the tool passesover the pads it bridges the pads together with solder. The head,however, at its trailing edge utilizes a non-solder wettable blade whichallows the solder surface tension to be broken. This leaves a welldefined amount of solder on each pad. The solder which is deposited is afunction of both the height of the non-wettable blade as well as thegeometry of the wettable area of the pad. In the case of this invention,because the pads are initially bridged together under the tool, and havean excess of solder on them, critical alignment of the tool is notrequired. Rather, the solder distributes itself evenly across the padafter having the blade pass over. Unlike screening techniques which areinherently limited by dimensional constraint and alignment problems, thedimensions of the head itself will be made very small to accommodateisolated and/or narrow sections of populated circuit boards. Also, inaccordance with this invention it is possible to distribute solder onnarrow pads, for example having 10 mil wide lines and a 20 mil pitch atan acceptable rate of processing.

Referring to FIGS. 6A and 6B a second embodiment of this invention isdepicted. In this embodiment, the substrate 19 is the same as that ofthe first embodiment. The pads 21 are also the same. This embodimentdeparts from the first embodiment in that rather than extruding a solderfoot, a discrete solder ball 60 is produced from the tool 62. That toolcomprises an outer shell 64 having a series of annular chambers. Thecentral chamber 66 contains a heater 10 for heating of the mass ofliquid solder 13. An outer annular chamber 68 dispenses a hot forminggas over the surface.

A heater 70 is used to preheat the pads 21 prior to the deposition ofthe solder ball 60.

As the tool 62 moves in the direction of arrow in FIGS. 6A, the heater 7first passes over pad 21. The temperature of the pad is then elevated tothe point that it will receive a discrete liquid solder drop 60 and notresult in a cold solder joint. When the central portion of the toolcontaining the solder mass 13 passes over the pad 21, solder underpressure from the chamber 66 produces a discrete amount of solder in theform of a drop 60. That solder mass once it contacts the preheated pad21 is deposited thereon with the mass breaking off by movement of thetool. The result is illustrated in FIG. 6A relative to the pad 21" is auniform deposition of the solder 27 onto the pad 21".

This system may be used in a unidirectional technique wherein the toolturns always proceeding in the direction wherein the heater 70 precedesit or, in an omnidirectional mode wherein no discrete turning of thetool is required. That is, while the tool may change direction to followa predetermined pattern, under computer control, the tool itself neednot rotate.

Referring then, to FIGS. 7A and 7B two omnidirectional solder emittingheads are depicted. In FIG. 7A the tool 70 has a regular series of holes72. The spacing of those holes presented only for purposes ofillustration, it being understood that they will become compacted orspread so that the combined emission is one solder mass which emergesupon extrusion.

The head 74 in FIG. 7B utilizes symmetrical cruciform shape 76. In thisembodiment the solder is extruded as one mass which tends to merge atthe central axis of the tool to form the solder mass which is depictedschematically in FIG. 6A.

Two unidirectional embodiments are illustrated in FIGS. 7C and 7D. InFIG. 7C the tool 78 comprises a series of three holes 80 openings and at7D the tool 82 comprises a series of elongated slots 84. It is apparentthat in the tool of FIGS. 7C and 7D rotation is necessary as the toolitself changes direction so that holes 80 or slots 84 are alignedperpendicular to the direction of travel.

This embodiment does not employ the non-wettable blade to achieveseparation. Such is deemed necessary but, may be used as an option.Rather, complete separation occurs because a discrete solder mass isextruded from the solder column which is maintained under pressure. Inthat regard, the system schematic of FIG. 4 may be employed to theextent that the system is maintained under pressure utilizing solderequipment.

Referring now to FIG. 8 a third embodiment of the solder deposition toolof this invention is illustrated.

This tool is omnidirectional in terms of its use, that is it needs norotation to accomplish solder deposition on pads at various angles tothe tool. The retaining wall 90 for a small solder volume may employvarious shapes for the bottom edge 94 which serves as a non-wettableblade as in the embodiments previously discussed. A sharp edge is shownin FIG. 8 although a rounded or squared off version is also useable.

This tool also differs from the previous embodiments in that the soldervolume is not regulated by means of a pressurized gas volume, but iscreated and removed and discarded as needed during the tool use andmovement. At the beginning of a cycle, a thermally isolated gear wheel95 and pressure wheel 96 are used to deliver a known length of wiresolder 98 into the solder reservoir area by way of an orifice 100. Thissolder melts on the heater source 102, and flows to the bottom of thereservoir area, forming a small solder volume 92. The small soldervolume eliminates the requirement for a negative back pressure tomaintain the solder in the reservoir area during the reflow mode. Thisis due in part to the wetting action of the solder to the heater source102 surface, as well as the minimized force of gravity acting upon thereduced amount of solder in the reservoir.

As the tool moves in the desired direction, it delivers precise amountsof solder on the pads 21 which emerge from the trailing section of thetool by the method of disturbing the surface tension of the solder bymeans of a non-wettable blade, in this case edge 94 in a mannerdescribed in previous embodiments. In this case, the blade is circular,and comprises the bottom edge of the retaining wall 90, which accountsfor the omnidirectional capability of the tool. The retaining wall maybe fabricated of any non-wettable material, such as aluminum, quartz,ceramic, etc. At the end of a row, or at the end of a tool cycle whenthe tool is to be raised and relocated, the solder volume in thereservoir is removed prior to raising the tool.

At the end of a row, or at the end of a tool cycle when the tool is tobe raised relocated, the solder volume in the reservoir is removed priorto raising the tool. This is accomplished be means of applying vacuum(not shown) to the chamber to which the solder removal pipe 104 isattached. This pipe may be heated by various techniques, to keep thesolder from solidifying in the pipe. Shown is a heater comprises ofnichrome wire 106 wrapped around the pipe 104. The pipe 104 may be madeof electrically non-conductive material to simplify the electricalisolation of the nichrome wire, such as quartz. This solder removalscheme allows most of the solder to be removed from the reservoir, withthe remaining solder clinging to the heater tip. The tool may be liftedfrom the substrate without fear of solder falling to the work piece.

Developments and modifications of this invention may be practicedwithout departing from the essential scope thereof. For example, apiston may be employed for pressurization instead of gas under pressure.In the first embodiment optical, magnetic or thermal sensors may beemployed.

We claim:
 1. Apparatus of applying liquid solder to a series ofconductive surfaces comprising,an omnidirectional movable housing havinga reservoir for liquid solder and a dispensing outlet whereby solderflows from said reservoir and forms a discrete mass that contacts aconductive surface as a solder film, means to preheat said conductivesurface prior to contact by said discrete mass, and means to maintainsaid liquid solder in said reservoir under positive gas pressure.
 2. Theapparatus of claim 1 wherein said housing is movable omnidirectional andsaid dispensing outlet comprises a symmetrical array of ports.
 3. Theapparatus of claim 1 wherein said housing is movable omnidirectional andsaid dispensing outlet comprises a symmetrical port.
 4. The apparatus ofclaim 1 wherein said housing is movable along a line and said dispensingoutlet comprises a series of ports positioned perpendicular to the lineof movement.
 5. The system of claim 1 wherein said means to preheatcomprises an annular chamber disposed in said housing around saidreservoir, a source of gas under pressure and nozzle means to dispensesaid gas to surround said discrete mass of solder as it is emitted fromsaid dispensing outlet.
 6. The apparatus of claim 1 further comprisingmeans in said reservoir to heat said solder.
 7. The apparatus of claim 1wherein said means to preheat said conductive surface comprises a fluxpreheater attached to said housing.
 8. The apparatus of claim 7 whereinsaid flux preheater surrounds said housing.
 9. The apparatus of claim 7wherein said flux preheater is attached at a position ahead of saiddispensing outlet in a direction of movement.
 10. Apparatus of applyingliquid solder omnidirectional to a series of conductive surfacescomprising:a movable housing having a reservoir for solder and asymmetrical dispensing outlet whereby solder flows from said reservoirand forms a discrete mass that contacts a conductive surface as a solderfilm, an omnidirectional means for moving said housing, means to deliverwire solder to said reservoir, a heating element in said reservoir tomaintain said solder in a liquid from for release by said symmetricaldispensing outlet.
 11. The apparatus of claim 10 wherein saidsymmetrical dispensing outlet comprises an opening at the bottom of saidreservoir having a tapered wall terminating in an edge defining anomnidirectional non-wettable blade.
 12. The apparatus of claim 10further comprising a solder removal conduit connected to said reservoirand in fluid contact with liquid solder and, heater means to heat saidconduit to prevent solder entrained therein from hardening.
 13. Theapparatus of claim 10 wherein said means to deliver solder comprisessupply means to feed a predetermined amount of solder wire into saidreservoir and wherein said heater melts said solder wire to a liquidvolume for release.
 14. The apparatus of claim 13 wherein said supplymeans to feed comprises a pair of rollers to feed solder wire, and asupply orifice for guiding said wire into said reservoir.